1. Field of the Invention
The present invention relates to a solid electrolytic capacitor utilizing valve metal such as tantalum or niobium and particularly relates to a capacitor element to be incorporated in the capacitor. The present invention also relates to a method of making such a capacitor element.
2. Description of the Related Art
Conventionally, as disclosed in JP-A-H7-74062, for example, such a kind of capacitor elements is manufactured as follows.
First, valve metal powder such as tantalum powder and an anode wire made of valve metal are prepared. Then, the valve metal powder is compacted into a porous chip body in a manner such that an end portion of the anode wire is embedded in the chip body. Next, the chip body is sintered with a high temperature in a vacuum to join particles of the valve metal powder.
Subsequently, a dielectric film is formed on particle surfaces of the valve metal powder constituting the chip body, and then a solid electrolyte layer is formed on the dielectric film, and finally a cathode film is formed on the solid electrolyte layer, whereby a capacitor element is provided.
Similarly, JP-A-H9-223644 discloses another example of a manufacturing method of a capacitor element.
The above-described sintered chip body is required to be porous, in other words, to be formed to contain as many tiny holes as possible. For that purpose, before the sintering process, valve metal powder to form a chip body is prepared with a sintering inhibitor preliminarily added thereto as impurities. The sintering inhibitor contains, for example, at least one of the following: phosphorus, oxygen, carbon, nitrogen, hydrogen, iron, nickel, and silicon.
Employing such a sintering inhibitor can cause some disadvantages due to diffusion which occurs during the sintering process. The diffusion carries the sintering inhibitor into the anode wire at the above-described end portion which is covered by the chip body.
The diffused sintering inhibitor may concentrate at the covered end portion and produce segregation thereat, making the root portion of the anode wire extremely brittle. As a result, the anode wire may be formed likely to be folded or broken easily. This can involve a rise in rejection rate in manufacturing processes or incident rate of electrical open-circuiting at the anode wire during use of the solid electrolytic capacitor device.